1. Technical Field
This invention relates in general to electronic circuits and, more particularly, to a digitally controlled voltage controlled oscillator.
2. Description of the Related Art
Voltage controlled oscillators (VCOs) are used in a variety of electronic circuits. One particularly important application using VCOs is in mobile communications, where VCOs are used into the phase-locked loop system to generate the frequency with the desired precision and stability characteristic. VCOs are also used in a variety of other applications.
Typically, the VCO is formed on the circuit board using an LC tank including an inductor and a varactor. In order to ensure that the tuning range of the VCO is wide enough to cover the required bandwidth, and to compensate for the spread of other components, the tank formed on the circuit board is initially fabricated to cover a frequency range in excess of the optimum frequency range. During the manufacturing process, the tank is adjusted to cover an optimum frequency range by trimming the inductance
Trimming the tank is a relatively expensive procedure, particularly for mass produced circuits, such as cellular transceivers. Further, trimming procedures require that the tank be formed on the circuit board rather than integrating the VCO on a chipset. This further increases the cost of the circuit using the VCO.
Therefore, a need has arisen for a VCO with a wide frequency range that does not require physical trimming.
In the present invention, a voltage controlled oscillator comprises an inductive element, a variable capacitive element coupled to said inductive element and a bank of switched capacitors coupled to said inductive element and said variable capacitive element.
The present invention provides significant advantages over the prior art. First, the need to physically trim the inductor of the LC tank is eliminated. Second, the bank of switched capacitors can be set such that the VCO""s free running frequency is close to the desired frequency in order to reduce the tuning range required to the varactor. This provides for fast tuning of a phase lock loop incorporating the voltage controlled oscillator. Third, the capacitor bank can be incorporated on an integrated circuit, reducing the cost of implementing the voltage controlled oscillator. Fourth, the capacitors in the bank can have very small values, providing very precise tuning.
The present invention allows the effective tuning range of the VCO can remain broad, while the tuning range attributable to the varactor can remain small. Therefore, a high sensitivity is not required to achieve a broad tuning range. With a lower VCO sensitivity, better noise performances can be achieved and it is easier to integrate the varactor of the tank on an integrated circuit. Integration of RF (radio frequency) VCOs in systems for cellular applications, or other mass produced products, is important because of the significant cost and power savings from eliminating discrete components. Further, the noise immunity of the board is improved.